Molybdenum disilicide matrix composites reinforced with refractory metal fibers

ABSTRACT

A molybdenum disilicide composite comprising refractory metal reinforcing fibers and sufficient particulate silicon carbide, silicon nitride, boron nitride, or silica to modify the coefficient of thermal expansion of the matrix to match that of the fiber is found to have improved high temperature strength, creep resistance, toughness, and resistance to matrix cracking during thermal cycling.

The invention was made under a U.S. Government contract and theGovernment has rights herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to reinforced intermetallic matrix composites,and particularly to refractory metal fiber reinforced molybdenumdisilicide matrices having improved high temperature strength, creepresistance, and toughness.

2. Description of the Prior Art

Molybdenum disilicide is an intermetallic compound with a meltingtemperature in excess of 2000° C., excellent high temperature oxidationresistance, and high thermal conductivity. Several problems, however,limit the use of molybdenum disilicide as a high temperature material,such as insufficient high temperature strength, creep resistance, andtoughness. Accordingly, fiber reinforcement of molybdenum disilicide hasbeen attempted, using continuous high strength refractory metal fiberssuch as tungsten, and molybdenum. During thermal cycling, however,refractory metal fiber reinforced molybdenum disilicide matricesexperience cracking caused by thermal stresses resulting fromdifferences of thermal coefficients of expansion of the matrix and thereinforcing fiber. As disclosed herein, applicants have now formulated arefractory metal fiber reinforced molybdenum disilicide compositecapable of withstanding such thermal stresses, due to the presence of aparticulate material which modifies the coefficient of thermal expansionof the matrix.

Attempts have been made previously to improve the high temperaturecapability of molybdenum disilicide matrix materials, such as byaddition of silicon carbide whiskers. For example, Petrovic et al, inU.S. Pat. No. 4,927,792, disclose a molybdenum disilicide matrixcomposite which is reinforced with SiC whiskers throughout the matrix,to improve strength at high temperatures. The patentees' approach toovercoming matrix cracking during thermal cycling is to have the fibersin tension, and the surrounding matrix under compression, thus requiringa relatively high density of uniformly spaced fibers. Petrovic et al donot, however, suggest the inclusion of particulates to reduce thedifferences between the coefficient of thermal expansion of the MoSi₂matrix and the reinforcing fibers.

Petrovic et al, in U.S. Pat. No. 4,970,179, disclose a modified MoSi₂alloy matrix composite wherein the matrix contains from about 10 toabout 30 percent SiC in the form of whiskers or submicron powder. Inorder to achieve increased strength at high temperatures, a portion ofthe MoSi₂ in the matrix is replaced with one or more refractory metalsilicides, selected from tungsten disilicide, niobium disilicide,tantalum disilicide, molybdenum trisilicide, tungsten trisilicide, etc.Petrovic et al do not, however, suggest the inclusion of particulates toreduce the differences between the coefficient of thermal expansion ofthe MoSi₂ matrix and reinforcing fibers, and in fact do not suggest theinclusion of continuous refractory metal reinforcing fibers tostrengthen the matrix.

Washburn, in U.S. Pat. No. 5,045,237, discloses a refractory electricaldevice for use as a heating element, ignitor, and heat sensor, whichcontains fine powders of molybdenum disilicide, silicon carbide, andaluminum nitride which are sintered or hot pressed into rigidstructures. The patent does not teach, however, the use of refractorymetal reinforcing fibers.

Agarwal et al, in U.S. Pat. No. 4,935,118, disclose a self-heated oxygensensor package having a heating element comprising silicon carbide,silicon nitride, or molybdenum disilicide, or mixtures thereof. Thereference teaches the addition of silicon nitride to avoid falsereadings of oxygen content, but makes no disclosure of modifying thermalexpansion coefficients or adding refractory metal reinforcing fibers.

Schrewelius, in U.S. Pat. No. 4,016,313, discloses a heat resistantmaterial for use in kilns, and attempts to overcome decreased strengthdue to oxidation by filling the pores of the silicon carbide matrixmaterial with an impregnate containing molybdenum disilicide andsilicon. The reference, however, does not attempt to strengthen thematrix by the addition of refractory metal fibers, or to modify thethermal expansion coefficient of the matrix.

In summary, while the prior art has disclosed the addition ofparticulate materials to molybdenum disilicide matrices to modify thehigh temperature properties thereof, or the use of reinforcing fibers inmatrices, the references have not taught molybdenum disilicide matriceshaving modified coefficients of thermal expansion which thereby reducestress between the matrices and continuous refractory metal reinforcingfibers encompassed therein. Thus, the references have not overcome theproblem of stress induced by thermal cycling of refractory metal fiberreinforced molybdenum disilicide matrices.

SUMMARY OF THE INVENTION

The present invention comprises a method for the production of amolybdenum disilicide matrix capable of repeated thermal cycling, havingimproved high strength properties. It is an object of the presentinvention to provide a method to alter the coefficient of thermalexpansion of a molybdenum disilicide matrix to more closely match thatof the refractory metal fiber utilized as a reinforcing means.

It is also an object of the present invention to provide a method forproducing a high temperature intermetallic composite comprising a matrixof molybdenum disilicide, a particulate inclusion to modify thecoefficient of thermal expansion of said matrix, and a continuousrefractory metal reinforcing fiber. It is a further purpose of thisinvention to provide a molybdenum disilicide matrix encompassing athermal expansion coefficient modifying amount of a particulatematerial, and a reinforcing refractory metal fiber.

Thus, the present invention comprises the addition to a molybdenumdisilicide matrix of from about 15 to about 60 percent by volume of aparticulate modifying agent selected from submicron powders, platelets,and whiskers of silicon carbide, silicon nitride, boron nitride, orsilica, or mixtures thereof, the composite further comprising from about5 to about 60 volume percent of a continuous reinforcing fiber selectedfrom tungsten, molybdenum, niobium, tantalum, and the alloys of suchmetals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Molybdenum disilicide is a promising intermetallic compound for hightemperature structural applications, particularly in oxidizingatmospheres. However, molybdenum disilicide by itself lacks sufficienthigh temperature strength, creep resistance, and toughness, andtherefore requires reinforcement with high strength fibers, such astungsten, molybdenum, niobium, titanium, or alloys of such refractorymetals. Such fiber reinforced molybdenum disilicide materials, however,are subject to microcracking induced by thermal expansion mismatchduring thermal cycling.

It has now been discovered that this problem may be overcome bymodifying the coefficient of thermal expansion of the molybdenumdisilicide matrix to closely approximate that of the refractory metalreinforcing fiber employed. This may be accomplished by the addition tothe matrix of a suitable amount of particulate silicon carbide, siliconnitride, boron nitride, or silica.

The molybdenum disilicide matrix materials suitable for use in thepresent invention comprise the commercially available powders ofessentially pure molybdenum disilicide or molybdenum disilicidecontaining other silicide alloying materials, such as tungstendisilicide and/or rhenium disilicide and other small alloying additions.The matrix material should be utilized as a powder, having a particlesize of from about 5 to about 14 microns, and preferably about 9 micronsin diameter.

Commercially available refractory metal fibers, selected from tungsten,molybdenum, niobium, tantalum, and the alloys of such metals, may beutilized to act as the matrix reinforcement. The preferred fibers aretungsten alloys, such as tungsten-rhenium (up to about 50 weight percentRh), tungsten-hafnium carbide (up to about 4 weight percent HfC),tungsten-thoria (from about 1 to about 4 weight percent ThO₂),tungsten-rhenium-hafnium carbide, potassium doped tungsten, andmolybdenum alloys such as potassium doped molybdenum, molybdenum-rhenium(up to about 50 weight percent Rh), and molybdenum-hafnium carbide (upto about 4 weight percent HfC). Suitable fibers include W-1ThO₂,W-1.5ThO₂, W-3Re, W-5Re, W-25Re, W-0.35HfC, W-4Re-0.35HfC, K doped W, Kdoped Mo, Mo-0.6HfC-0.5C, Nb-28W-2Hf-0.06C, and Nb-11W-28Ta-1Zr. Thefiber may be present in the final composite in an amount appropriate toachieve the desired strengthening, such as from about 5 to about 60volume percent of the final composite, preferably from about 20 to about40 volume percent, and most preferably from about 30 to about 40 volumepercent.

Such fibers, having a diameter of, for example, about 125 microns, maybe drawn through a binder-powder mixture to apply a coating ofmolybdenum disilicide and particulate modifying agent, in a bondingresin, to the surface of the fibers. The thus coated reinforcing fibersmay then be assembled in layers, e.g. in the form of a tape, or inbundles, which may then be grouped as desired and subjected toappropriate fabrication techniques to remove the binder and form thedesired composites. It is also appropriate, when utilizing refractorymetal reinforcing fibers, to apply a protective barrier coating to thesurface thereof to prevent silicon diffusion in the interface betweenthe fiber and the molybdenum disilicide matrix. Such diffusion barriercoatings may be applied by conventional techniques, such as chemicalvapor deposition or sputtering, and include aluminum oxide (Al₂ O₃),yttria (Y₂ O₃), YAG (3Y₂ O₃ -5Al₂ O₃), mullite (3Al₂ O₃ -2SiO₂),zirconia (ZrO₂), titanium carbide (TiC), tantalum carbide (TaC), andzirconium carbide (ZrC).

It has been found that the coefficient of thermal expansion ofmolybdenum disilicide may be altered by the addition thereto ofparticulate silicon carbide, silicon nitride, boron nitride, or silica,or mixtures thereof. Such modifying material may be added to themolybdenum disilicide in the form of submicron powder, platelets having,for example, an aspect ratio of up to about 50:1, or whiskers having,for example, an aspect ratio of up to about 100:1. The amount ofparticulate to be utilized will be dependent upon the coefficient ofthermal expansion of the refractory metal reinforcing fiber to beemployed, the amount of refractory metal fiber to be present in thefinal composite, the diameter of said refractory metal fiber, and thetemperature ranges through which the desired composite is to be cycled.The specific proportion of particulate to be present in the matrix ofthe composite may be determined experimentally, or theoretically, toachieve the desired reduction in cracking. In general, however, theparticulate may be present in amounts approximating from about 15 toabout 60 volume percent of the matrix. The preferred amount ofparticulate preferably constitutes from about 20 to about 60 volumepercent of the matrix, and most preferably from about 30 to about 60volume percent. It is noted that silicon nitride has, from about 200° F.to about 2600° F., a lower coefficient of thermal expansion than siliconcarbide, and may thus be anticipated to have a greater modifying effectupon the coefficient of thermal expansion of molybdenum disilicide thanan equal amount of silicon carbide. It is possible to achieve very closeapproximations of the means coefficient of thermal expansion of thereinforcing fiber over a given temperature range by mixing varyingamounts and forms of silicon carbide, silicon nitride, boron nitride,and silica particulates in the matrix.

EXAMPLE 1

A tungsten alloy fiber reinforced molybdenum disilicide matrix compositewas prepared, formulated to provide 20 volume percent silicon carbideplatelets and 20 volume percent silicon powder in the matrix. A tungstenalloy fiber, comprising 3 weight percent rhenium, i.e. W-3Re, obtainedfrom GTE Products, and bearing a diffusion barrier of alumina, wascoated with a binder-powder mixture containing MoSi₂ powder obtainedfrom Herman Stark Co., and having an average particle size of about 9microns, SiC platelet obtained from C-Axis Technologies, and SiC powderobtained from Lonza Corporation. The matrix and modifier particulateswere suspended in a NeoCryl B-700 methacrylate polymer commerciallyavailable from ICI Resins, dissolved in a solvent comprising 70 volumepercent heptane and 30 volume percent acetone, to which was added 2weight percent ethylene glycol. The binder and the particulate materialswere present in a ratio of 2:7, although this ratio could be varied tosuit the amount of particulate matrix and modifier it is desired toimpart to the fiber surface. Alternatively, a Rhoplex methylmethacrylate polymer binder, available from Rhom & Haas, or othersuitable binders, could be used in place of the NeoCryl. After passagethrough the binder-powder material, the fiber was passed through afurnace at about 165° F. to fuse the binder resin, thus forming atungsten alloy fiber having a sheath of organic resin containing MoSi₂and SiC particulates. This fiber was then wound about a drum adapted toreceive the web of fibers and deposit of binder-powder, and formed intoa tape. Sections of tape formed by this technique were then assembledinto a stack and subjected to hot pressing at about 2250° F. for about1.5 hours at about 4 Ksi pressure. The thus consolidated composite, withthe resin burned out, was then subjected to hot isostatic pressing atabout 2510° F. for about 1 hour at about 35 Ksi pressure, to form afully consolidated composite. It was found upon testing of the compositethat the addition of the particulate silicon carbide phase to the matrixresulted in a substantial reduction in composite matrix expansion and inmatrix cracking when subjected to cycling between high and lowtemperatures. A substantial improvement in creep rate was noted at 2190°F. and 10 Ksi pressure, with the composite demonstrating a creep rate of4.12×10⁻⁸ per second, and the monolithic molybdenum disilicide having acreep rate of 1.79×10⁻⁷ per second, a factor of 4 improvement. Fracturetoughness of the composite measured 16 MPa√m, while the monolithicmolybdenum disilicide measured 4.5 Mpa√m, a factor of 3 improvement.

EXAMPLE 2

A molybdenum disilicide matrix composite similar to that of Example 1was prepared, utilizing a potassium doped molybdenum reinforcing fiber,coated with an aluminum oxide diffusion barrier. It was found that theaddition of the particulate phase to the matrix was effective inlowering the coefficient of thermal expansion of the matrix andpreventing matrix cracking. Further, a factor of 70 improvement in creeprate was obtained, with the composite yielding a creep rate of 7×10⁻⁸per second, as compared to the monolithic measuring 5×10⁻⁶ per second at2010° F. and 14.5 Ksi pressure. A 25 fold improvement in fracturetoughness was also measured, with the composite yielding 125 Mpa√m andthe monolith 4.5 MPa√m.

Similar results are obtained utilizing 40 volume percent silicon nitridein a molybdenum disilicide matrix reinforced by a tungsten alloy fiber.

It is to be understood that alternative methods for the preparation ofthe composites themselves are available, and that the above descriptionof the present invention is susceptible to considerable modification,change, and adaptation by those skilled in the art, and that suchmodifications, changes, and adaptations, are to be considered within thescope of the present invention, which is set forth by the appendedclaims.

What is claimed is:
 1. A refractory metal fiber reinforced molybdenumdisilicide composite comprising molybdenum disilicide, refractory metalfiber, and sufficient particulate modifying agent selected from thegroup consisting of silicon carbide, silicon nitride, boron nitride,silica and mixtures thereof, to decrease the coefficient of thermalexpansion of said molybdenum disilicide.
 2. The composite of claim 1,wherein said modifying agent comprises from about 15 to about 60 percentby volume of the matrix.
 3. The composite of claim 2, wherein saidreinforcing fiber is selected from the group consisting of tungsten,molybdenum, niobium, tantalum, and alloys thereof.
 4. The composite ofclaim 3, wherein said fiber is selected from tungsten, molybdenum, andalloys thereof.
 5. The composite of claim 4, wherein said fiber istungsten, and said modifying agent comprises from about 15 to about 60percent by volume of said matrix.
 6. The composite of claim 4, whereinsaid modifying agent comprises from about 20 to about 60 percent byvolume of said matrix.
 7. A molybdenum disilicide composite comprising arefractory metal reinforcing fiber, and from about 15 to about 60 volumepercent of a particulate modifying agent selected from silicon carbide,silicon nitride, boron nitride, silica, and mixtures thereof, based uponthe matrix.
 8. The composite of claim 7, wherein said fiber comprisesfrom about 5 to about 60 volume percent of said composite.
 9. Thecomposite of claim 8, wherein said fiber is selected from the groupconsisting of tungsten, molybdenum, and alloys thereof, and comprisesfrom about 30 to about 40 volume percent of said composite.
 10. Thecomposite of claim 8, wherein said fiber is tungsten or an alloythereof, and said modifying agent is silicon carbide.
 11. The compositeof claim 8, wherein said fiber is molybdenum or an alloy thereof, andsaid modifying agent is silicon carbide.
 12. The composite of claim 8,wherein said fiber is tungsten or an alloy thereof, and said modifyingagent is silicon nitride.
 13. The composite of claim 8, wherein saidfiber is molybdenum or an alloy thereof, and said modifying agent issilicon nitride.